The present invention relates to a magnetic scale and a method of manufacturing the same, and more particularly to a magnetic scale and a method of manufacturing the same by coating a ferromagnetic base member by flame spraying, plating, or the like, and heating the coated surface at spaced intervals with a laser beam to produce modified portions of a nonmagnetic material, so that magnetic and nonmagnetic areas are alternately defined.
Heretofore, magnetic scales are widely used in production sites or the like. One prior magnetic scale includes a ferromagnetic base member having a plurality of equally spaced grooves each having a width ranging from 1 to 2 mm and a depth ranging from 0.2 to 0.3 mm, the grooves being filled with chromium, for example, by plating. In the plating process, a chromium layer tends to concentrate on the outer corners of the grooves, but is apt to be less deposited on the bottoms of the grooves. As a result, the plated layer develops projections on the groove corners. The distance between the tip ends of the projections and the upper surfaces of the plated layer on the groove bottoms is increased to the extent which would make it difficult to plate the grooves. Therefore, it is customary to plate the grooves while removing the projections developed on the groove corners. The developed projections must however be removed repeatedly several times, sometimes up to ten times, until the grooves which are 0.2 to 0.3 mm deep are entirely filled up. Accordingly, the plating process is quite time-consuming and hence highly costly.
Another conventional process is to plate a nonmagnetic base member with nickel containing phosphorus, applying a laser beam to the plated surface layer at equally spaced intervals to heat the plated layer up to about 300.degree. C. for thereby modifying the nickel layer into a ferromagnetic layer. Since the base member is plated over its entire surface, no such a complex procedure as required by the above prior plating process is necessary. However, the thickness of the plated layer is limited to 0.2 mm because of the bonding capability of the layer and the production efficiency.
It is confirmed that when a magnetic scale is employed to measure the amount of displacement of a plunger for forcing molten metal into a die cavity in a die casting machine, an electric output issued from a detector for detecting the amount of displacement is proportional to the thickness of the plated layer.
Therefore, the electric output cannot be increased substantially by the layer thickness of about 0.2 mm. Moreover, inasmuch as the plated layer is locally modified into the ferromagnetic layer by laser beam heating, when the magnetic scale in use is subjected to frictional heat or heated due for example to a machine failure, the characteristics of the magnetic scale, i.e., the electric output is lowered, and the magnetic scale fails to operate normally especially when its temperature approaches 300.degree. C. Therefore, the magnetic scale only finds use in limited applications.